w should be the actual particle weight

[RemiLehe]

In the code before this PR, the particle quantity w represented different things:

• For non-ionizable particles: it was the weight (i.e. number of physical particle per macroparticle) times the charge.
• For ionizable particles: it was the weight times the ionization level, times the elementary charge e. This also implied that we had to keep an additional quantity (neutral_weight) to store the actual number of physical particle per macroparticle, inside the Ionizer class.

This is confusing, and therefore the current pull request sets w to be the actual weight (i.e. number of physical particle per macroparticle) for both non-ionizable particles and ionizable particles.

• For non-ionizable, this implies that w has to be multiplied by the charge inside the deposition kernel (this is a minor cost)
• For ionizable particles, w now plays the part that neutral_weight used to play. And therefore, inside the Ionizer class, neutral_weight is replaced by a new array w_times_level, which is only used for the deposition kernel, as the effective weight.

Note that a lot of the changes involve replacing neutral_weight by w_times_level in all the MPI communications. The code for the MPI communications of these optional quantities (e.g. w_times_level, ionization_level, etc.) is quite ugly, and should at some point be replaced by something better.

PS: In fact, w_times_level does not need to be communicated via MPI, since it can be recalculated at each exchange_period, from w and ionization_level. I kept the MPI communications mostly to minimize the changes in the code that this PR produces.

[MKirchen]

Nice PR! Good changes!

[RemiLehe]

Thanks for reviewing!